High-precision optical guidance systems for high-speed air vehicles (as well as other multi-wavelength systems such as satellite spectrographs) can require the simultaneous use of several cryogenic optical devices operating in different frequency ranges, which must be carefully aligned with one another. The frequency and power characteristics of these systems are widely divergent.
The extreme precision with which the systems must operate makes it necessary to align them very accurately with respect to each other so that their information may be properly correlated. This requirement presented several problems in the prior art. To begin with, the traditional way of making dewars involved complete fabrication of the dewar itself, followed by the bonding of the glass envelope to a metallic mounting base by a bonding agent such as epoxy. Alignment of the detectors with the reference plane provided by the mounting base was difficult for two reasons: firstly, the detector was no longer visible within the silvered dewar after fabrication, so that alignment has to be done by placing the hollow coldfinger stem onto a mandrel; and secondly, the poor tolerances of the glass envelope and the unpredictable shrinkage of the epoxy presented a substantial potential of misalignment.
Another problem of the prior art arose from the fact that replacement of one of several separate dewars after the original manufacture of the equipment required a careful realignment of the apparatus. This can be done fairly routinely in a factory environment but is difficult to do at maintenance facilities in the field. It would therefore be desirable to mount all the detectors in a single, easily factory-alignable part, particularly inasmuch as such a single-piece construction would allow the use of a single cryogenic element with a resulting substantial reduction in weight, size and cost.
Unfortunately, consolidation of several detectors in a single dewar has not been practical in the past because the lowest power detector array was subject to being overwhelmed by stray optical and electromagnetic interference from one or more of the other optical systems. Under these circumstances, serious electromagnetic interference was apt to occur. Consequently, complete physical separation of the detector systems was the only practical prior art approach.
The following publications are representative of the prior art in this field: U.S. Pat. No. 3,415,994, which shows a dual-element infrared detector; U.S. Pat. No. 3,851,173, which shows a cryogenic assembly for an infrared detector; U.S. Pat. No. 3,962,578, which shows cascaded photoelectric detector elements with an integral filter for two-color detection; U.S. Pat. No. 4,487,037, which shows a single-array dewar construction; U.S. Pat. No. 4,528,449, which shows a single-array dewar using metal subassemblies for improved serviceability; and published British application No. 2,082,763A, which shows a cascaded infrared detector with sensors of different composition.